1. Field of the Invention
The present invention concerns a method for determining the position in at least one spatial direction of a local coil on a patient positioning table or board within a magnetic resonance tomography scanner, in which a radio-frequency signal is emitted in volume in which a magnetic field gradient is applied in the appertaining spatial direction and a signal profile is measured along the magnetic field gradient by means of the appertaining local coil. The invention also concerns a corresponding control device for a magnetic resonance tomography apparatus for determining the position of a local coil according to such a method as well as a magnetic resonance tomography apparatus with such a control device.
2. Description of the Prior Art
Modern magnetic resonance examination systems normally operate with a number of various antennas (called coils in the following) to emit radio-frequency pulses for nuclear magnetic resonance excitation and/or to receive the induced magnetic resonance signals. A magnetic resonance system normally have a larger whole-body coil installed in a fixed manner in the apparatus. The whole-body coil is typically cylindrically arranged around the patient acquisition space (for example with what is known as a birdcage structure) in which the patient is situated on the patient positioning table during the measurement. Furthermore, one or more small local coils or surface coils are frequently used in a scanner. The local coils serve to acquire detailed images of body parts that are located relatively close to the body surface. For this purpose, the local coils are applied directly on the point of the patient at which the region to be examined is located. Given a use of such a local coil, in many cases transmission is made with the whole-body coil (as a transmission coil) and the induced magnetic resonance signals are received with the local coil (as a reception coil).
For the magnetic resonance examination, it is important to know the exact position of the local coils that are employed relative to the patient table, and therewith relative to the patient. In principle, with the patient table moved out of the imaging volume, it is possible to measure the position of the coils manually using rulers, measuring sticks, markings on the patient table etc. Such a measurement is notably complicated and additionally entails the danger that the measured positions are associated with the wrong coils, in particular given a use of a number of coils or given the use of coil arrays formed of a number of coils. In principle, it is therefore simpler and safer to automatically determine the position of the local coils in the framework of a magnetic resonance measurement
Such an automatic determination of the position of a local coil on a patient table in a specific spatial direction can ensue according to the initially-described method, by applying a magnetic field gradient in the appertaining spatial direction in which the position should be determined. A radio-frequency signal is then emitted by the whole-body coil or with a local coil, and a reception signal profile is measured along the magnetic field gradient by means of the appertaining local coil whose position should be established. The position of the local coil in the appertaining spatial direction is finally determined with the aid of this signal profile.
Such a method is specified in German PS 196 53 535. The reception signals are received by, apart from the local coil to be localized, an antenna with a homogenous sensitivity over the entire measurement space, for example by the whole-body coil. The intensity values received by the local coil are normalized with the intensity values received by the antenna with homogenous sensitivity. A maximum point obtained from the spatial curve of these normalized intensity values is then acquired as a position value for the local coil. Since, due to the noise portions in the measurement signals, this spatial determination can be relatively error-prone, in German OS 102 07 736 it is proposed to adapt a function (that is approximated to the form of the sensitivity profile of the local coil) to the spatial curve of the intensity values, and to use on the maximum point of this function as the position of the local coil.
A problem in the determination of the coil position according to one of these known methods occurs in the particularly significant determination of the position of the coil in the longitudinal direction of the patient table—typically designated as the z-direction. The possibility exists that the coil is located all the way at the edge of or outside of the scanner, and therewith outside of the magnetic field, since the patient table can be adjusted in the z-direction and is normally only partially located within the scanner. However, if a local coil is located directly on the edge of the magnetic resonance scanner, the coil profile varies (for example) unpredictably and chaotically. If the local coil is located outside of the scanner, a relevant coil profile can no longer be measured at all. This leads to a completely wrong position being determined for such local coils with the typical position determination methods.